The present invention relates generally to magneto flywheels utilized in small gasoline engines. More particularly, the invention relates to a magneto flywheel having an improved key.
Small gasoline engines, such as are frequently incorporated into string trimmers, chain saws, lawn mowers and other such devices, typically utilize a magneto ignition system. As is well known, a magneto ignition system will selectively produce a spark at the engine spark plug due to the magnetic cooperation between a stationary coil assembly and a rotating permanent magnet assembly.
Typically, the magnet assembly of the magneto ignition system will be carried by the engine""s flywheel. Often, the flywheel will also include cooling vanes to circulate cooling air over the engine as the flywheel is rotated. The flywheel itself is generally mounted to a rotating shaft attached at one end to the engine""s crank arm. The magnet assembly is positioned at a particular angular location about the axis of the shaft to ensure correct ignition timing.
In a typical construction, the flywheel is formed of cast aluminum. The hub of the flywheel defines a conical or xe2x80x9ctaperedxe2x80x9d bore which mates with a complementary surface machined on the shaft. The tapered bore of the flywheel""s hub includes a key located in a keyway defined on the tapered portion of the shaft. A securing element, such as a nut, maintains the flywheel in position on the shaft taper.
The flywheel is normally attached to the shaft at a level of tightening torque deemed to be optimum in a particular situation. For example, a typical cast aluminum flywheel having a 3.5 inch outer diameter may be attached to the shaft by a tightening torque of about 16 to 18 foot-pounds. Occasionally, however, a flywheel may be subjected to much higher tightening torques (e.g., 50 foot-pounds or more). The higher tightening torques may be due to overtightening at the factory, or may be caused in some cases by the design of downstream components such as clutches. Whatever the cause, severe overtightening of the flywheel has often resulted in flywheel breakage.
The present invention recognizes and addresses the foregoing disadvantages, and others of prior art constructions and methods. Accordingly, it is an object of the present invention to provide a novel flywheel for use in a small gasoline engine.
It is a particular object of the present invention to provide a flywheel for a small gasoline engine having an improved key.
It is a more particular object of the present invention to provide a flywheel for use in a small gasoline engine which is less susceptible to breakage due to overtightening.
It is also an object of the present invention to provide a flywheel for use in a small gasoline engine which may be efficiently manufactured.
Some of these objects are achieved by a flywheel mountable to a drive shaft of a small internal combustion engine. The flywheel comprises a flywheel body including a central hub portion and an integral wheel portion. The hub portion of the flywheel body defines an axial bore extending therethrough in which the engine""s drive shaft is received when the flywheel is mounted thereon. An integral key is located in the bore for receipt in a corresponding keyway defined in the drive shaft. The key has opposed side walls extending directly from an inside surface of the through bore about a radius located at the respective base thereof.
The key defines a top surface extending between the opposed side walls, which may have an arcuate profile in the axial direction of the bore. Preferably, the key in such embodiments will have an axial extent less than that of the axial bore. For example, the arcuate profile of the top surface may terminate at both ends thereof by convergence into the inside surface of the axial bore. The axial bore itself will often be configured having a conical configuration.
Other objects of the present invention are achieved by a flywheel mountable to a drive shaft of a small internal combustion engine. The flywheel comprises a flywheel body including a central hub portion and an integral wheel portion. The hub portion of the flywheel body defines an axial bore extending therethrough in which the engine""s drive shaft is received when the flywheel is mounted thereon.
The flywheel further comprises an integral key located in the axial bore for receipt in a corresponding keyway defined in the drive shaft. The key has opposed side walls and a top surface extending therebetween. The side walls of the key extend directly from an inside surface of the through bore. In addition, the top surface of the key has an arcuate profile in the axial direction of the axial bore.
In addition, the flywheel includes a magneto assembly located adjacent a periphery of the wheel portion of the flywheel body. A plurality of vane members, configured to pass air around the flywheel during operative rotation thereof, are integrally formed on the wheel portion of the flywheel body. At least one counterweight assembly is located adjacent a periphery of the wheel portion so as to balance the magneto assembly during operative rotation of the flywheel.
Other objects, features and aspects of the present invention are discussed in greater detail below.